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Electrochemistry
Conductance in Electrolytic Solutions * An electrolytic solution is a solution through which electric current can be passed . * Conductance (G) is the reciprocal of resistance . * Conductivity (k) is the reciprocal of resistivity . * Molar Conductivity(Λ) of a solution is defined as the electrolytic conductivity k divided by the molar concentration © of the dissolved electrolyte . Λ= k/C * Λ = kV = 1000 k / C. (Concentration in terms of Molarity) * Molar Conductivity of strong electrolytes varies as Λ = Λo - a √C * α = Λ / Λ o ... K = Λ2C / Λ0(Λ 0 - Λ ) * Measurement of Conductivity * Determination of Cell Constant * Determination of Conductivity : k = b/R * Equivalent Conductivity = Conductivity / No. of Valence Electrons Kohlrausch Law Kohlrausch law states that at infinite dilution , each ion migrates independently of its co-ion and makes its own contribution of an electrolyte irrespective of the nature of other ion . for AxBy ; Λo = xλ+o + yλ-o Electric Conduction Metallic Conduction : '''The current flowing through metals due to potential difference . '''Ionic or Electrolytic Conduction : '''The current flowing through a solution due to the migration of cations and anions . Electrodes '''Cathodes Redution takes place at Cathode . Anodes Oxidation takes place at Anode . Electrochemical Cells Electrolysis Electrolytic Cells Electrolysis is defined as passing of electric current through a solution to make a chemical reaction occur spontaneously . An electrolytic cell provides potential difference for carrying out electrolysis . Electrolysis of NaCl Fused''' NaCl''' Reduction : Na+ + e- -------> Na Oxidation : 2Cl- --------> Cl2 + 2e- Net Reaction : 2Na+ + 2Cl- -------> 2 Na + Cl2 Cl2 has been collected at the anode . Na has been collected at the cathode . Aqueous NaCl Reduction : Na+ + e- ------> Na 2H2O + 2e- ------> H2 + 2OH- Oxidation : Cl- ------> Cl + e- 2H2O ------> O2 + 4H+ + 4e- Net Reaction : 2Cl- + H2O -------> Cl2 + H2 + 2OH- H2 gas is liberated at cathode . Cl2 gas is liberated at anode . OH - ions are are formed in reduction half reaction . Na remains unatracted NaCl is converted to NaOH . This , we can conclude that Sodium cannot be obtained by electrolysis of Aqueous solution . Like Sodium , none of the s-block elements can be obtained by electrolysis of their aqueous solutions . Faraday's Laws of Electrolysis Faraday's First Law of Electrolysis : The amount of substance that undergoes oxidation or reduction is directly proportional to the amount of current passed through it . w α q w α It w = zIt , where w is the weight deposited at the electrode , z is the electro chemical equivalent z = grams deposited by 1 Coloumb of electricity Faraday's Second Law of Electrolysis : When same amount of electricity is passed through different cells containing different electrolytes and arranged in series , the amount of substances oxidized or reduced at the respective electrodes are directly proportional to their equivalent masses . W1 / W2 = E1 / E2 1 Farad = 6.022 x 1023 x 1.6022 x 10-19 = 96500 Coloumb mol -1 1 Farad of electricity , deposits 1 equivalent weight of the substance Electrochemical Equivalent and Equivalent Weight w / eq. wt = z / eq. wt equivalent weight = electrochemical equivalent x 96500 Galvanic or Voltaic Cells In a galvanic or voltaic cell , a spontaneous chemical reaction generates electric current . Salt Bridge A salt bridge is needed in Voltaic cells , to provide electrical contact between two solutions . It also prevents the diffusion of ions into each other . In most galvanic cells , a salt bridge is needed for reaction to occur . A salt bridge is a narrow tube , filled with agar agar gel and KCl (Cl ions) . Daniel Cell Types of Electrodes Electrode Potential & Cell Potentials Cell Potential The cell potential , also called the Electro-Motive Force , measures the tendency of the cell reaction to occur . The standard cell potential is the cell potential when reactants & products are in the standard states . The cell potential is related to Gibbs Energy changes to the of the cell reactions by the equations Δ G = - nFE Electrode Potential The electrode potential is the measure of tendency of an element to undergo reduction . Nersnt Reaction Reference Electrode Common Types of Cells Dry Cell Lead Accumulators Nickel - Cadmium Cell Fuel Cells Hydrogen - Oxygen Fuel Cell Concentration Cells Electrochemical Series Electrochical Series is an arrangement of electrodes in decreasing order of their standard reduction potentials . Higher the reduction potential , greater is the tendency to get reduced , better is the oxidizing agent . Lower the reduction potential , greater is the tendency to get oxidized , better is the reducing agent . Some important comparisons : Elec Corrosion Corrosion is an electrochemical process in which metal (Fe) is oxidized in an anode region of metal surface and oxygen is reduced in the cathode region . Test Yourself ! You are now ready to take a test on this chapter . Take a 30 minute test here : Test : Electrochemistry Category:Chemistry